The threshold of perception of field flicker in a television display system is a function of the flicker frequency and the brightness of the display. Over the years displays have increased in brightness to the point where flicker is noticeable even in relatively high field rate systems (e.g., NTSC-60 Hz) and clearly objectionable in lower field rate systems (e.g., PAL-50 Hz). A solution to this problem is to double the field rate and double (or quadruple) the line rate of displayed images as described, for example, by Lord et al. in U.S. Pat. No. 4,322,750 entitled TELEVISION DISPLAY SYSTM which issued Mar. 30, 1982. In an example of the Lord et al. system, a video input signal is stored in a field memory. Each stored field is recovered or "read" twice from the memory and displayed on a display scanned at double the line rate and double the field rate of the incoming video signal.
In the Lord et al. system, the memory read and write clocks are derived from the output of a sync separator circuit. It would be advantageous, in certain applications, to lock the memory read and write clocks to multiples of the color subcarrier reference frequency to thereby simplify chroma demodulation and to minimize the possibility of cross-color effects and other undesirable artifacts in displayed images.